A known developing cartridge includes a bag-like developer container (hereinafter, referred to as a “developer bag”) within its housing. The developer bag stores a developer therein. The developer bag has an outlet for releasing the developer therethrough. The outlet is closed by a film-like seal member to seal the developer bag. The seal member is fixed to a shaft of an agitator, and is removed or peeled from the developer bag by rotation of the agitator. The removal of the seal member thus allows the developer to flow from the developer bag through the outlet.